This invention relates generally to dynamoelectric machines and more particularly to rotors and rotor laminations for use in a dynamoelectric machine.
Rotors for dynamoelectric machines of the type having permanent magnets mounted on the outer surface of the rotor often require containment structures to prevent the magnets from being thrown off the rotor by centrifugal force at the operating speed of the dynamoelectric machine. Presently used forms of containment include non-magnetic structures (e.g., stainless steel sleeves, wire wraps, glass wraps and plastic over-molding) mounted or formed on the outer surface of the rotor, or holes formed in the rotor itself which receive the permanent magnets. Use of non-magnetic containment structures requires substantial additional manufacturing steps beyond stamping rotor laminations and stacking them to form a rotor core. The structure must be attached to or formed on the rotor core after it comes out of the forming die. This additional step, as well as the additional material required for the containment structure, significantly add to the cost of producing the motor. Moreover, in the case of stainless steel containment sleeves extending unbroken along the length of the rotor core, substantial eddy currents are conducted producing corresponding losses in the dynamoelectric machine.
Mounting the permanent magnets within the rotor core, which is made of a magnetic material, creates the problem of leakage of magnetic flux from the permanent magnet, causing a corresponding loss in efficiency of the dynamoelectric machine. Flux leakage occurs when lines of flux from one pole of the magnet pass through rotor material located between the permanent magnet and stator to the other pole of the permanent magnet without crossing the air gap and passing through the stator. In applications where the motor is running frequently, the motor inefficiency constitutes a substantial portion of overall inefficiency of the apparatus driven by the motor, flux leakage losses are of significant concern. Another source of efficiency loss for dynamoelectric machines in which permanent magnets are held within the rotor cores is reluctance components caused by having a relative large amount of magnetic rotor material between the permanent magnet and the stator which varies in width across the radially outer face of the permanent magnet.
Dynamoelectric machines can experience vibrations caused by torque pulsations. The problem is particularly acute in the context of variable speed motors which are operated over a wide range of speeds. The frequency of the vibrations changes with the speed of the motor, and the vibrations are transmitted from the rotor to the rotor shaft, and thence to the apparatus connected to the shaft. When such motors are used, for instance, in a blower unit with a squirrel cage fan in a heating and cooling system, the vibrations caused by torque pulsations and transmitted from the rotor to the fan through the connecting shaft cause the fan (or other components of the blower unit) to ring when the frequency of the pulsations corresponds to a natural vibration frequency of the fan. The audible noise produced by the blower unit is highly undesirable. The problem is not limited to permanent magnet rotors, but also occurs in other types of rotors such as switch reluctance rotors.
It is known to dampen the vibration in the rotor core by introduction of a rubber vibration absorber between the rotor (or part of the rotor) and the rotor shaft. More specifically, it is known to separate the rotor core into generally annular inner and outer members connected together solely by two elastomeric rings. The elastomeric rings absorb and dampen the vibrations caused by torque pulsations. However, it is difficult to dimension the rings closely enough to position the outer surface of the rotor in closely spaced relation with the stator in the stator bore. Moreover, the shape of the elastomeric ring can change over time as a result of the vibrations so that the concentricity between the inner and outer members changes, as does the spacing between the outer periphery of the rotor and stator.
In some applications, a dynamoelectric machine is connected to a shaft which is out of balance in rotation about its axis. For example a compressor shaft may be out of balance in rotation because of cams on the shaft used to drive pistons in the compressor. The amount by which the shaft is out of balance is readily determined and can be counterbalanced. Heretofore, such counterbalancing has been carried out by application of counterweights to the rotor core. The counterweights are an additional material cost, and their application to the rotor core is an additional manufacturing step which increases the cost of producing the dynamoelectric machine.